1. Field of the Invention
The present invention relates to an integrated process for the treatment of the residual solutions resulting from anodization plants producing aluminium or aluminium alloy parts, where use is made of an aqueous solution of sulphuric acid in the anodization bath and of a lye wash of caustic soda (i.e. NaOH) in the pickling bath, when this latter stage is included in the operation. The invention likewise relates to an installation enabling the process to be carried out.
2. Description of the Prior Art
It is known that anodization, also termed anodic oxidation, is an electrochemical process for the treatment of surfaces of aluminium and its alloys by the formation of protective coatings of variable thickness and texture consisting of oxides of the base metal. The main stages are those of degreasing the parts to be treated, pickling them, anodizing them and then sealing by means of hot water or steam the layer of oxide obtained by anodization. In certain cases, however, e.g. in a surface treatment intended for the production of shiny components of aluminium or aluminium alloy, the pickling is dispensed with. In between the successive bath treatments mentioned, the parts are vigorously rinsed with water.
The composition of the successive baths may vary from one production chain to another, but for numerous applications, particularly for the anodization of matt strips and sections, the pickling is generally effected in an aqueous bath of caustic soda, while the anodization bath is essentially an aqueous solution of sulphuric acid.
It is precisely to the treatment of the residual solutions containing caustic soda (if pickling is effected) and sulphuric acid respectively, in varying proportions, that the present invention relates. One particular variant thereof applies to cases in which no pickling is carried out.
It is well known (cf. S. Wernick and R. Pipper--"The surface treatment and finishing of aluminium and its alloys"--R. Draper, Teddington, Chap.21) that the rinse waters, abundant but diluted, have to be treated separately from the concentrated pickling and anodization solutions, by combining them, adjusting the pH value (e.g. by the addition of caustic soda or of hydrochloric acid), decantation and draining off or possibly, partial recycling. Precipitated aluminium hydroxide is not toxic; it can, therefore, be separated by filtration or some similar means and evacuated to a dumping site.
The concentrated residual water emanating directly from the pickling procedure, if any, and from the anodization must, therefore, be treated independently of the rinse water (cf. S. Wernick and R. Pipper--cited above) and the present invention relates more particularly to the treatment of the concentrated water and to the installations required for this purpose.
The problem can be summarized by assuming, for example, that the output of waste water from pickling contains 50 g/l of caustic soda and 100 g/l of sodium aluminate (as well as traces of the sodium gluconate used as a stabilizing agent for the aluminate) while the output of waste water from the anodization contains, for example, 200 g/l of sulphuric acid and 70 g/l of aluminium sulphate. The amount drawn off from the pickling vat must naturally be kept to a minimum, but must, nevertheless, be sufficient to ensure that the concentration of sodium aluminate in this vat will not exceed the threshold above which an uncontrolled precipitation of aluminium hydroxide will occur. Similarly, the quantity drawn off from the anodization tank must be sufficient to ensure that the concentration of aluminium sulphate will not exceed the threshold beyond which flaws would occur in the layer of oxide obtained by anodization.
Numerous processes have already been proposed, either for the treatment of the waste pickling water by itself or for that of the pickling water and anodization water in one integrated process.
According to Japanese Pat. No. 75-151,929 by Hashimote Hirozaku and Marushima Norio, for instance, the two outputs are mixed with lime, in such a way as to obtain, after precipitation, filtration and reaction at 1200.degree. C., an additive for mortar or cement.
According to Japanese Pat. No. 76-34,563 by Hayashi Toshio, it is possible, by proceeding in stages, to recycle the caustic soda. By mixing the two outputs, the aluminium hydroxide is precipitated and a solution of sodium sulphate formed. Filtration is then effected, thus forming a cake of aluminium hydroxide, which will be eliminated, and a filtrate, to which lime is added. This results in the precipitation of gypsum (eliminated after filtration) and a solution containing most of the caustic soda initially introduced into the bath.
According to Japanese Pat. No. 74-130,907 by Tashire Hiroshi, a sodium aluminate solution is prepared by alternating reactions and filtrations of the two outputs. This solution is subsequently reacted with a cobalt salt or magnesium salt in such a way as to precipitate the corresponding aluminate. After filtration, drying and reaction above 1000.degree. C., a spinel is obtained which can be used in the manufacture of refractories.
According to Japanese Pat. No. 74-107,023 by Tashire Hiroshi, Yamashite Masato and Harada Noboue, the process is confined to the treatment of the waste pickling solution. By the addition of a limewash, a calcium aluminate is precipitated from which an aluminous cement is drawn off by filtration, dried, heated to 1200.degree. C. and crushed. It is clear that the filtrate contains the caustic soda, which as a general principle is capable of being recycled.
Finally, according to Japanese Pat. No. 533,111 by Professor Tajima, the two outputs are treated separately. To the residual pickling solution is added sodium silicate in order to precipitate a double silicate of aluminium and sodium and to regenerate, in principle completely, the initial bath of caustic soda. Furthermore, the aluminium sulphate of the anodization bath is precipitated in the form of ammonium alum or potassium alum by the addition of ammonium sulphate or potassium sulphate. The unconverted sulphuric acid is capable of being recycled.
These various processes suffer from certain drawbacks, particularly from the fact that one or more supplementary reactants have to be purchased in order to effect the reactions, that the saleability of the resulting products is not certain, that when used in industry the recycled outputs may be contaminated by the new products and that the precipitation might continue to take place in the tanks. Furthermore, some of the above methods involve the use of furnaces operating at very high temperatures, which increase both the installation costs and the operating costs.